The present invention relates to a system for controlling the supercharging pressure of an internal combustion engine having a turbocharger.
An internal combustion engine for an automobile is provided with a turbocharger driven by a small capacity exhaust gas turbine, so that the turbocharger may operate at a low engine speed. In such an engine, the supercharging pressure increases excessively with an increase of engine speed, with the risk of breakdown of the engine at high engine speed. In order to avoid this, various systems have been proposed. A conventional system comprises a bypass provided around the exhaust-gas turbine, a waste gate valve in the bypass, and an actuator for operating the waste gate valve. The actuator comprises a diaphragm operatively connected to the waste gate valve, the diaphragm partitioning the actuator into chambers at atmospheric pressure and at the supercharging pressure respectively. The diaphragm is deflected by the difference between the atmospheric pressure and the supercharging pressure as the supercharging pressure increases. Thus, the actuator operates to open the waste gate valve to discharge the exhaust gases through the bypass when the engine speed is higher than a predetermined speed. Accordingly, the increase of the supercharging pressure is limited.
However, since the atmospheric pressure is low at high altitude and therefore the actuator operates at a low supercharging pressure, the supercharging pressure is held at a lower maximum value than at low altitude, for the same engine speed. Therefore, an increase of the engine output by the turbocharger cannot be obtained at high altitude.